Digital radio communications are subject to errors caused by broadband noise and burst noise phenomena. A known means of improving the receiving sensitivity of a digital signal in the presence of such phenomena, which has been used successfully, is word encoding using block codes, which is effective against random errors as caused by broadband noise, and dispersion coding for protection against random burst errors such as those caused by Rayleigh fading. Two examples of dispersion coding known to one of ordinary skill in the art are convolutional coding and two dimensional interleaving of a block of words. Two dimensional interleaving of a block of words is accomplished by arranging a number of words, wherein the words are typically encoded by a block code, and the symbols of the words are transmitted sequentially, a symbol from each word, until all the symbols of all the words have been transmitted.
In simulcast radio communication systems, radio signals are transmitted simultaneously from more than one transmitter. At a selective call receiver used in a simulcast system, the signal intercepted by the antenna is likely to be comprised of a signal from more than one transmitter. There are overlap areas in such a system, which are geographic areas of the system where the received signal is comprised of two or more signals of substantially similar strength. When carrier frequencies of the transmitters in a simulcast system are precisely controlled to have exactly the same frequencies, standing wave patterns are set up in the overlap areas. The resultant signal in such an overlap area will vary at a rate equivalent to carrier frequency differences between the transmitters, which may be very slow in an accurately and precisely controlled system. This causes a situation where selective call receivers located in these areas will have very poor probability of recovering a signal due to signal cancellation and distortion. For this reason, it is typical in simulcast systems to intentionally offset the carrier frequencies of transmitters having overlapping coverage areas, for example, by 50 Hz, so as to eliminate the very slowly changing standing waves. This results is periodic bursts of errors, at a rate corresponding to the offset frequency, rather than long periods of time when reception is impossible, which allows messages to be received and understood in the areas where otherwise the probability of signal recovery is poor.
The simulcast situation caused by interfering radio waves is an example of a general case of interference where the interfering signals are similar and have known offset frequencies. Geographic reuse of channels, a technique common in cellular telephone service, is another example of the general case.
In simulcast and geographic reuse systems as described above, data signals which are encoded with typical burst and random error protection schemes as described above do not always achieve nearly as much improvement as with random noise and Rayleigh fading environments. A typical forward error correction code will operate satisfactorily only when the received bit error rate (BER) is less than about 2 percent. However, the received BER may be 10 percent or even higher in simulcast overlap areas.
"Soft" decision decoding is often used to improve the performance of forward error correction codes. When signal quality is estimated for each received symbol, the decoder can use this information to determine which of several possible symbols is most likely in error.
The effectiveness of soft decision decoding is dependent on the accuracy of the signal quality estimates. Methods based on received signal strength are not reliable when interference is present due to simulcasting or an independent signal present on the same channel. The received signal strength may be high at the same time that the interference causes destructive distortion of the received signal. Thus, what is needed is a means to improve the protection of digital signals against the very destructive periodic error bursts which occur in simulcasting as well as against errors resulting from random noise and random burst errors.